In a conventional XY table, a first motor is used to drive a first stage of the XY table and a second motor is used to drive a second stage of the XY table. The second stage is usually mounted onto the first stage. A load, such as that comprising a bond head and bonding member, is attached to the second stage and moves with it. Commonly, the first stage is called an X stage which moves the XY table in an X-axis and the second stage is called a Y stage which moves the XY table in a Y-axis. A combination of movement of the X and Y stages result in the positioning of the load in an X-Y plane.
The load, together with the X and Y stages of the XY table, comprise a moving mass of the XY table which mass has a combined weight that is concentrated in a center of gravity approximately located centrally in the XY table structure when it is in an equilibrium position. At the equilibrium position, when the X stage is driven, the orientation of the X stage is such that a motor driving the X stage (“X motor”) generates a force that is aligned with the center of gravity of the mass when the Y stage is in a home or central position.
However, a problem with mounting the Y stage onto the X stage is that the said center of gravity of the moving mass shifts when the Y stage moves. This will happen no matter how well the driving force is aligned with the center of gravity at the equilibrium position. Once the center of gravity moves, the driving force will not be aligned with the center of gravity. When the driving force is not aligned with the center of gravity, a turning moment is generated that is equal to the driving force multiplied by the distance that the center of gravity is shifted from the equilibrium position. This undesirable turning moment causes the moving mass to vibrate as the mass is moved, thereby increasing the performance differential and increasing the amount of time required for accurate positioning at a given location, since such vibration will cause some positional offset. The further the Y stage moves away from the equilibrium position, the greater the turning moment that induces vibration.
Problems associated with displacement of the center of gravity as an object or load is moved are identified in U.S. Pat. No. 5,844,664 for “Positioning Device with a Force Actuator, System for Compensating Center-of Gravity Displacements, and Lithographic Drive provided with such a Positioning Device”. The patent discloses a positioning device and a method to compensate for shifts in the center of gravity. A plurality of vertically-arranged force actuator systems generate compensation forces to balance an object table when the center of gravity moves as the object table is displaced. Although a balancing force is generated as a passive response to the movement of the center of gravity, there is no disclosure of how to align the driving force to the center of gravity as the object table is moved, that would improve the driving efficiency and performance.